F O
C
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Talking Satellites
I
n oil exploration, knowing where you are plays just as
[490 ft] and Transit provided a fix only about once an hour to
important a role as knowing where the oil is. For posi-
locations near the equator.
tioning and navigation data, explorationists are now
Throughout the transportation industry, research is under-
looking skyward toward an orbiting constellation of satellites
way to harness GPS for managing air traffic, train routing,
to execute seismic surveys, position rigs and locate machinery.
and ship navigation, for dispatching delivery truck fleets and
The Navstar Global Positioning System (GPS), introduced
reducing gridlock. Scientists use GPS to track continental
in 1983 as a tool for enhancing US military capabilities, is a
drift (using sophisticated analysis techniques), predict earth-
satellite-based radio navigation system (next page, top).
quakes and monitor changes in the atmosphere caused by
When fully operational at the end of 1993, GPS will comprise
the greenhouse effect. For oil patch applications, GPS is
24 satellites, including three spares, each orbiting the earth
more accessible, more accurate and less expensive than
every 12 hours at an altitude of 20,200 km [12,500 miles].
conventional land-based radio navigation systems.
Equipped with four atomic clocks, each satellite broadcasts
Accessibility to GPS is worldwide and continuous—a
precise time, satellite position and condition data 24 hours a
boon to underdeveloped areas and far offshore regions
day. At least five satellites will be visible to users anywhere
where no other effective system exists. GPS can accommo-
in the world. So far, 18 satellites are in orbit.
date an infinite number of users whereas land-based sys-
The fundamental measurement of GPS is the time
required for a signal to travel from a satellite to the
tems used in marine seismic restrict the number of users.
For oil exploration, most of the cost savings from GPS
receiver—often a portable, hand-held device—which gives
stem from its global coverage. Crews on land conducting a
the satellite-receiver distance. Measurements collected
seismic survey or positioning a rig must sight between sur-
simultaneously from four satellites, each distinguished by a
vey points, which often requires massive and costly clearing
unique code, are processed at the receiver to determine in
of vegetation. GPS demands only enough clearance to view
real time the receiver’s longitude, latitude, altitude and veloc-
the satellites from each survey point. In marine seismic, the
ity, if the receiver is moving. Error is 3 to 100 meters [10 to
operator frequently foots the expense of establishing the
330 feet], depending on the amount of error correction
navigation chain, operating it and demobilizing equipment
applied during processing and type of signal being processed.
when the survey is completed. Even in locations with a land-
GPS is not the first satellite-based system used for navigation. The Transit system, developed by the US Navy in
1960, contained fewer satellites in tighter orbits. To calculate
based system in place, operators rely on time-consuming
and costly licensing negotiations with local authorities.
With conventional radio navigation, operators at the start
a position with Transit, a receiver measured the Doppler
of a survey install job-specific equipment that takes at least a
effect, or frequency shift, detected in signals emitted by a
day to calibrate and verify. GPS receivers, once installed, are
satellite of known trajectory. Errors were as high as 150 m
permanent, and calibration and verification takes minutes.
Conventional radio navigation may also be limited by inter-
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Oilfield Review
The GPS system, with worldwide, continuous coverage,
provides useful navigation and
positioning data for land and
marine seismic operations.
ference from nearby vessels and aircraft, the shape of the
shoreline, elevation of onshore beacons and vegetation that
deflects signals. These limitations are absent from GPS.
The global accessibility of GPS proves a double-edged
sword to the originator of the system, the US Department of
Defense (DOD). To maintain US military advantage with GPS,
DOD offers two GPS services: precise positioning service
(PPS) and standard positioning service (SPS). PPS signals
can fix a position to within 10 m [33 ft], but are encrypted to
prevent unauthorized access. Only users who satisfy national
security requirements have access to PPS signals. The SPS
DGPS improves the accuracy
of GPS measurements by
comparing the known coordinates of a nearby reference
with its GPS fix.
signals, on the other hand, are available to everyone but
have been degraded through a technique known as selective
availability (SA). With SA activated, accuracy plummets an
order of magnitude to 100 m, unacceptable for surveying
and marine navigation.
SA limitations can be circumvented, however, by a technique called differential GPS or DGPS, first developed in the
late 1980s by scientists studying the shape of the earth
(above, right ). DGPS compares known coordinates of a
October 1992
71
nearby fixed reference receiver with the receiver’s GPS fix.
The error from the GPS signal can then be transmitted to the
Air gun
array
user, who applies the corrections to the received signals or
computed position. In the North Sea, for example, compaFront
buoy
nies offering DGPS services claim an accuracy of 3 to 5 m
[10 to 16 ft] at distances up to 2000 km [1240 miles] from a
reference station.
Much controversy surrounds the continued US enforcement of SA, especially during peacetime. Civilian users point
to the success of DGPS and the development of a similar
satellite system by the former Soviet Union called GLONASS.
However, SA is here to stay as long as the current GPS system affords the US and its allies a military advantage, even
by forcing hostile nations to develop DGPS capabilities.
From an oil industry perspective, DGPS would be utilized
with or without SA to meet the stringent positioning requirements of seismic surveys (accuracy to within 5 m).
Placement of GPS receivers
on a marine seismic survey
and (inset) a GPS receiver on
a front buoy.
The most elaborate use of GPS in oil exploration is marine
seismic, which marries the new technology with conventional navigation and positioning techniques. During a survey, GPS receivers may be located on the survey vessels, the
front buoys and the tail buoys (right ). Satellite-to-vessel and
satellite-to-buoy ranges are computed to derive the position
of the front and tail buoys relative to the vessel. Acoustic
positioning devices called transponders are located on the
Tail buoy
survey vessels, source arrays, streamers and tail buoys. (Transponders are still needed because GPS receivers do not oper-
GPS receiver
Magnetic compass
ate underwater.) The position of the transponder relative to a
survey vessel or buoy is determined by measuring the time
quently, DGPS will not replace, but rather integrate with,
it takes acoustic pulses emitted from one transponder to
existing systems. Use of DGPS as a primary positioning sys-
reach another. In addition, laser ranges measure the distance
tem has grown in the North Sea because its greater reliability
between the vessel and source arrays, and compasses
minimizes the risk of operational failure. Improving software
spaced along each cable monitor the streamer shape.
that can integrate DGPS data with existing measurements
Typically, a backup positioning system is desired in a
marine seismic survey in case the primary one fails. Conse-
will secure the cost savings promised by the new technology.
—TAL
Acknowledgements and Further Reading
For help in preparing this focus, thanks to Erik Vigen,
GECO-PRAKLA R&E, Sandvika, Norway and Bruce
King, GECO-PRAKLA, Stavanger, Norway.
For further reading:
Jensen MHB: “Quality Control for Differential GPS in
Offshore Oil and Gas Exploration, ” GPS World 3, no.
8 (September 1992): 36-48.
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